1. Field of the Invention
The present invention pertains to a power amplifier for mobile communications which adaptively changes the frequency band among a plurality of frequency bands. In particular, it pertains to a feed forward amplifier for multiple frequency bands which collectively amplifies a plurality of frequency bands.
2. Description of Related Art
The base configuration of a conventionally used feed forward amplifier is shown in FIG. 1. The feed forward amplifier includes two signal processing circuits. One is a distortion detection circuit 150 and the other is a distortion elimination circuit 151. Distortion detection circuit 150 is composed of a main amplifier signal path 153 and a linear signal path 154. Distortion elimination circuit 151 is composed of a main signal path 158 and a distortion injection path 159. Main amplifier signal path 153 (also called a vector adjustment path) is composed of a vector adjuster 155 and a main amplifier 156. Vector adjuster 155 is composed of a variable phase shifter 155a and a variable attenuator 155b. Linear signal path 154 is composed of delay lines. Also, main signal path 158 is composed of delay lines. Distortion injection path 159 (also called a vector adjustment path) is composed of a vector adjuster 200 and an auxiliary amplifier 201. Vector adjuster 200 is composed of a variable phase shifter 200a and a variable attenuator 200b. Here, a divider 152, a power combiner/divider 157, and a combiner 202 are simple lossless power dividers and power combiners composed of transformer circuits and hybrid circuits.
First, an explanation of the basic operation of the feed forward amplifier will be given. The signal input into the feed forward amplifier is divided into main amplifier signal path 153 and linear signal path 154 by means of divider 152. At this point, variable phase shifter 155a and variable attenuator 155b of main amplifier signal path 153 are adjusted so that the signals of main amplifier signal path 153 and linear signal path 154 have equal amplitude and opposite phase. As methods for bringing the paths to opposite phases, there is the method wherein divider 152 or power combiner/divider 157 sets a phase shift appropriately between the input and output terminals or the method wherein main amplifier 156 inverts the phase.
Since distortion detection circuit 150 is configured in this way, power combiner/divider 157 can output the differential component of the signal passing through main amplifier signal path 153 and the signal passing through linear signal path 154. This differential component is precisely the distortion component generated in main amplifier 156. Due to this fact, the block from divider 152 to power combiner/divider 157 shown in FIG. 1 is called a distortion detection circuit.
Next, an explanation regarding distortion elimination circuit 151 will be given. The output of distortion elimination circuit 150 is divided, via power combiner/divider 157, into main signal path 158 and distortion injection path 159. The output of main amplifier 156 from main amplifier signal path 153 (the signal passing through main amplifier signal path 153) is input into main signal path 158. Also, the differential component of main amplifier 156 detected in distortion detection circuit 150 (the differential component of the signal passing through main amplifier signal path 153 and the signal passing through linear signal path 154) is input into distortion injection path 159. As for variable phase shifter 200a and variable attenuator 200b of distortion injection path 159, the distortion components of the signal passing through main signal path 158 and the signal passing through distortion injection path 159 are adjusted so as have equal amplitude and opposite phase. By making an adjustment in this way, combiner 202 can combine the signal passing through main amplifier signal path 153 with the distortion component of main amplifier 156 having equal amplitude and opposite phase. And then, combiner 202 outputs a signal in which the distortion components of the whole amplifier are cancelled. Further, even if it is a matter of common knowledge, a linear amplifier is used as an auxiliary amplifier in order to eliminate the distortion component generated in the main amplifier used in a feed forward amplifier. The aforementioned operation is an ideal operation of a feed forward amplifier. In practice, it is not simple to completely maintain a balance of the distortion detection circuit and the distortion elimination circuit. Also, even if tentatively the initial settings are perfect, since the properties of the amplifier change due to fluctuations in ambient temperature, power supply, and the like, it is extremely difficult to preserve an excellent balance which is stable over time.
As methods for maintaining a highly accurate balance of the distortion detection circuit and the distortion elimination circuit of this feed forward amplifier, there is known an self-adjusting method using a pilot signal. E.g., there exist the Japanese Patent Application Laid-Open Publication No. 1 (1989)-198809 (Patent Reference 1) and the like. As devices putting these methods into practical use, there is known the article “Extremely Low-Distortion Multi-Carrier Amplifier For Mobile Communication Systems—Self-adjusting Feed-Forward Amplifier (SAFF-A)” by Toshio Nojima and Shoichi Narahashi, Institute of Electronics, Information, and Communication Engineers, Wireless Communication Systems Society, RCS90-4, 1990 (Non-patent Reference 1). These feed forward amplifiers were put into practice in the 800 MHz band and the 1.5 GHz band of the PDC (Personal Digital Cellular) mobile communications standard in Japan. This kind of feed forward amplifier is generally designed and adjusted to amplify separately for each frequency band.
The feed forward amplifiers of Japanese Patent Application Laid-Open Publication No. 2000-223961 (Patent Reference 2) and Japanese Patent Application Laid-Open Publication No. 2001-284975 (Patent Reference 3) fragment a single transmission band, e.g. 20 MHz inside the 2 GHz band, by means of a plurality of band pass filters, and amplify the fragmented and extracted signals. And then, the same amplifier compensates, separately for each fragmented frequency, the amplitude divergence and the phase divergence generated in the amplifier to raise the distortion compensation accuracy.
In the radio systems developed this far, a single system in accordance with any one of PDC, GSM (Global System for Mobile communications), IMT-2000 (International Mobile Telecommunications 2000), and the like, was used. As against this, there is the technology of carrying out a transfer to software of some functionality of radio devices so that it becomes possible for a single hardware to handle a plurality of radio systems. If it is possible for a single hardware to handle a plurality of radio systems, the user can use the mobile communication environment without any awareness of the radio system or the core network in the background thereof. However, a single hardware actually handling a plurality of radio systems is something that has not reached implementation.
Also, it can be considered that, for each region or operator, the services offered with the radio system will be different and that the radio systems will also gradually become diversified. For this reason, it can be considered that, in the future, there will arise a need to make radio systems coexist which are optimal for each purpose, at the same time and in the same place.
As methods of using these plural radio systems, there is the multiband radio system. This radio system adaptively changes the frequency band used or the number of frequency bands used in response to the propagation environment and the traffic conditions. Also, in order to ensure a prescribed transmission quality or transmission volume, multiband transmission using frequency bands not in use is effective. Consequently, in a multiband radio system, in order to ensure the transmission quality or transmission volume to be guaranteed by the same radio system, the number of frequency bands is changed. Moreover, changes are also carried out in the same way within the same frequency band. Further, a multiband radio system, in case there coexist frequency bands used by several operators, can raise the frequency utilization efficiency by carrying out adaptive control using available frequency bands by means of interference recognition technology, frequency sharing technology, interference cancellation technology, produced interference reduction and avoidance technology, multiband control technology, and the like.
The feed forward amplifier is used as a linear amplifier for base stations handling multiband radio systems like this. However, in case the plural frequency bands to be amplified are widely separated, compared to the bandwidth of each frequency band, the adjustment levels of the variable phase shifter and the variable attenuator for keeping the balance of the distortion detection circuit and the distortion elimination circuit within a designated range vary with the frequency band to be amplified, because the electrical length of the delay line for each frequency band differs.
To put it in concrete terms, in case a delay line is used in common for all frequency bands, there is, due to the frequency differences of the input signals, ordinarily a need for the setting value of the vector adjuster to track a signal rotating with the angular velocity of the frequency difference. However, in the vector adjusters developed this far, it has not been possible to track a signal rotating at a velocity like that. Also, as for the vector adjusters discussed this far, it has not been possible to simultaneously set an optimal amplitude and phase, with respect to plural input signals, for structural reasons.
E.g., in case 800 MHz band and 1.5 GHz band signals are input into the same vector adjuster, it is possible to carry out optimal vector adjustment with respect to any one of the frequency bands. However, it is not possible to carry out optimal vector adjustment which tracks a frequency difference of 700 MHz. Consequently, the conventional feed forward amplifier has not been able to simultaneously amplify the 800 MHz band signal and the 1.5 GHz band signal at or below a prescribed distortion compensation level.
As a method of resolving this, a dual-band feed forward amplifier is proposed in the article “A Dual-Band Feed-Forward Amplifier” by Yasunori Suzuki and Shoichi Narahashi, the 2005 General Meeting of the Institute of Electronics Information and Communication Engineers, C-2-2, March 2005 (Non-patent Reference 2). With this configuration, there is proposed, for each frequency band, a vector adjuster having a band extraction means. In other words, this dual-band feed forward amplifier extracts the signal of the vector adjusted frequency band from the input signals of two frequency bands by means of a filter provided in a pre-stage of the vector adjuster. And then, vector adjustment is carried out for each frequency band. This dual-band feed forward amplifier configuration is capable of distortion compensation in a plurality of frequency bands. Further, the compensated band is fixed by the filter.
In multiband radio systems having a plurality of transmission bands, it can be considered to change the frequency band due to the service situation of the radio system, interference of other radio systems, and the like. However, as mentioned above, the bandwidth of the distortion compensation of the feed forward amplifier is determined by the adjustment accuracy of each loop of the distortion detection circuit and the distortion elimination circuit. Consequently, in the conventional feed forward amplifier, the adjustment of distortion compensation could not be made to correspond with the frequency band changes. Also, it was not possible for the conventional dual-band feed forward amplifier in which the distortion compensated frequency band was fixed to adaptively change the operating frequency. For a feed forward amplifier used over a long time, the change in frequency band accompanies repairs or a change of the feed forward amplifier in the base station. Consequently, an enormous amount of labor and time is required to readjust a large number of feed forward amplifiers. A feed forward amplifier configuration making this kind of labor and time expense unnecessary was required.
E.g., in case, for a dual-band feed forward amplifier which simultaneously compensates the distortion of a signal in a frequency band f1 and a signal in a frequency band f2, the frequency band was changed from f2 to f3, it has not been possible to simultaneously compensate the distortion of the signal in frequency band f1 and the signal in frequency band f3. This was so because loop adjustment by the frequency difference of f1 and f3 was not possible, as mentioned above, due to the fact that the operating frequencies of a conventional dual-band feed forward amplifier are fixed.
Also, there can be considered the method of providing, in the dual-band feed forward amplifier, fixed filter and vector adjusters handling all the frequency bands that may be thought to be available for future service. However, having fixed filters and vector adjusters able to handle all the frequency bands amounts to having fixed filters and vector adjusters which are not used something which runs counter to configuring a cost-effective feed forward amplifier. There was demanded a feed forward amplifier with no need for the exchange of constituent parts and having no redundancy to accompany in this way the changes in frequency band or the increase and/or decrease in the number of carrier waves.